Active imaging-antenna arrangements for the millimeter and microwave range are becoming increasingly important as a result of their diversity of possible uses in the fields of medicine and industry.
Such antenna arrangements comprise a plurality of individual transmission and reception antennas, whereas each antenna transmits a signal in succession, which is reflected from an object, and the reflected signal is received by all of the reception antennas. This is performed either at a single frequency or stepwise at different frequencies. In this context, both the amplitude and the phase of the received signal are measured. Using a digital beam-forming algorithm (digital beam-forming), it is possible to calculate back from this to the corresponding object points, which have reflected the received signal, and accordingly, an image of the entire reflected body can be prepared.
Such an antenna arrangement is described, for example, in an article in “IEEE Transactions on Microwave Theory and Techniques”, Vol. 59, No. 12, pages 3567-3576, December 2011, entitled “A Novel Fully Electronic Active Real Time Imager Based on a Planar Multi-static Sparse Array”. To ensure that the focusing process for such an antenna arrangement functions with a digital beam-forming method, a well-defined phase must be present between the individual transmitter and receiver pair. In order to achieve this, a calibration of every transmitter/receiver pair is implemented. In an antenna device with hundreds or thousands of transmitter and receiver units, a measurement of the direct connection of every combination is not possible. Accordingly, the calibration of an antenna device which operates in the reflection mode using a calibration object which provides a simple and regular geometry is described in the above-named article. The measurements with the calibration object are then compared with simulation results or with previous reference measurements in order to calculate the systematic error in the antenna device. These investigated errors are determined for every transmitter/receiver pair, for all frequencies used. For this purpose, for example, a metal plate positioned at a predetermined distance is used as the calibration object.
One major problem with this calibration method is that the relative position between the calibration object and the antenna device must itself be observed very accurately. Since these antenna devices operate in the millimeter-wave range, the spacing distance between the calibration object and the antenna device, as well as the position and the orientation of the calibration object must be known with greater accuracy than one wavelength, whereas the spacing distance between the antenna device and the calibration object corresponds to hundreds of wavelengths. With a maximum the phase error of 5° and a signal wavelength of 100 GHz, an accuracy of approximately 40 μm in the spacing distance is therefore required. Such accuracy can only be achieved with very high mechanical costs.